Classification of supraventricular and ventricular cardiac rhythms using cross channel timing algorithm

ABSTRACT

A system and method for classifying cardiac complexes sensed during a tachycardia episode. A first cardiac signal and a second cardiac signal are sensed, where the first cardiac signal has a voltage. A first cardiac complex and a second cardiac complex of a cardiac cycle are detected in the first and second cardiac signal, respectively. A predetermined alignment feature is identified in the second cardiac complex. A datum is defined, or positioned, at a specified interval from the predetermined alignment feature of the second cardiac complex. Voltage values are then measured from the first cardiac complex at each of two or more measurement intervals from the datum. The voltage values are then compared voltage values measured from NSR cardiac complexes to classify the first cardiac complex is either a ventricular tachycardia complex or a supraventricular tachycardiac complex.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.11/077,653 filed Mar. 11, 2005, which is a division of U.S. patentapplication Ser. No. 10/202,297, filed Jul. 23, 2002, now issued as U.S.Pat. No. 6,889,081, which is a division of U.S. patent application Ser.No. 09/352,056, filed Jul. 14, 1999, now issued as U.S. Pat. No.6,449,503, the specifications of which are incorporated herein byreference.

FIELD OF THE INVENTION

The subject matter relates generally to medical devices and moreparticularly to classification of sensed cardiac complexes.

BACKGROUND

Effective, efficient ventricular pumping action depends on propercardiac function. Proper cardiac function, in turn, relies on thesynchronized contractions of the heart at regular intervals. When normalcardiac rhythm is initiated at the sinoatrial node, the heart is said tobe in sinus rhythm. However, when the heart experiences irregularitiesin its coordinated contraction, due to electrophysiologic disturbancescaused by a disease process or from an electrical disturbance, the heartis denoted to be arrhythmic. The resulting cardiac arrhythmia impairscardiac efficiency and can be a potential life threatening event.

Cardiac arrhythmias occurring in the atrial region of the heart arecalled supraventricular tachyarrhythmias (SVTs). Cardiac arrhythmiasoccurring in the ventricular region of the heart are called ventriculartachyarrhythmias (VTs). SVTs and VTs are morphologically andphysiologically distinct events. VTs take many forms, includingventricular fibrillation and ventricular tachycardia. Ventricularfibrillation is a condition denoted by extremely rapid, nonsynchronouscontractions of the ventricles. This condition is fatal unless the heartis returned to sinus rhythm within a few minutes.

Ventricular tachycardia are conditions denoted by a rapid heart beat,150 to 250 beats per minute, that has its origin in some abnormallocation within the ventricular myocardium. The abnormal location istypically results from damage to the ventricular myocardium from amyocardial infarction. Ventricular tachycardia can quickly degenerateinto ventricular fibrillation.

SVTs also take many forms, including atrial fibrillation and atrialflutter. Both conditions are characterized by rapid uncoordinatedcontractions of the atria. Besides being hemodynamically inefficient,the rapid contractions of the atria can also adversely effect theventricular rate. This occurs when the aberrant contractile impulse inthe atria are transmitted to the ventricles. It is then possible for theaberrant atrial signals to induce VTs, such as a ventriculartachycardia.

Implantable cardioverter/defibrillators (ICDs) have been established asan effective treatment for patients with serious ventriculartachyarrhythmias. The first generation of ICDs relied exclusively onventricular rate sensing for tachyarrhythmia detection. Specificity toSVT was, however, often compromised, especially when the ventricularresponse to SVT surpassed the patient's heart rate during VT. Thefrequency of inappropriate shocks with early generations of signalchamber ICDs ranged from 10-41% of the shocks. Detection enhancements,such as Sudden Onset and Stability of the cardiac rhythms, improvedspecificity in more modem ICDs. The introduction of dual chamberdefibrillators further improved upon the specificity to SVT withoutcompromising sensitivity to VT. unfortunately, some patients stillreceive inappropriate therapies for SVT, especially whenatria-to-ventricular conduction is 1:1.

Morphology-based algorithms have been proposed as a way ofdistinguishing VT from SVT. Many of these algorithms are templatematching algorithms which determine the type of tachycardia by comparingfeatures of the electrogram in question with an efficient representationof the patient's normal sinus rhythm (NSR) electrogram. The basis ofappropriate discrimination using template-matching algorithms are basedon the assumption that the morphology of ventricular depolarizationduring VT will be dissimilar to those during NSR. These algorithmsclassify cardiac complexes based on their morphological similarity tothe patient's NSR complexes using only one intracardiac electrogramchannel. In the process of comparing any two complexes, the algorithmlocates a fiducial point (e.g., the peak of the complex) to align thetwo complexes with respect to each other. This alignment has the sideeffect of positioning complexes such that they appear to be more similarthen they actually are. As a result, differentiating the two complexesbecomes more difficult.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art forproviding a reliable system of discriminating SVT induced ventriculartachycardia from malignant ventricular tachycardia which can provideeffective and reliable therapy to patients experiencing malignantventricular tachycardia.

SUMMARY OF THE INVENTION

The present subject matter is directed to a system and a method fordistinguishing between the occurrence of a ventricular tachycardia (VT)and a supraventricular tachycardia (SVT) during a tachycardia episode.Upon detecting a tachycardia episode, the system measures voltage valuesfrom predetermined positions along sensed cardiac signals. The voltagevalues are then compared to voltage values measured at the same relativepositions along model cardiac complexes. Using this comparison, thesystem is able to distinguish the underlying cause of a tachycardiaepisode as either being an SVT or as a VT.

Initially, a first model cardiac complex and a second model cardiaccomplex are detected, or sensed, in the first cardiac signal and thesecond cardiac signal, respectively. In one embodiment, the first andsecond model cardiac complexes are normal sinus rhythm (NSR) cardiaccomplexes sensed during normal sinus rhythm. Alternatively, the firstand second model cardiac complexes are cardiac complexes which areinduced by electrical pulses delivered to at least one supraventricularlocation of the heart.

As the second model cardiac complex is sensed, a predetermined alignmentfeature is identified. In one embodiment, the predetermined alignmentfeature of the second cardiac complex is a repeatably identifiableportion of the second cardiac complex, such as a maximum deflectionpoint of the second cardiac complex. The predetermined alignment featureis then used to define, or position, a datum at a specified intervalfrom the predetermined alignment feature. In one embodiment, the datumcan be thought of as a line, or a position, from which to make voltagemeasurements along the first cardiac signal during the first cardiaccomplex. In one embodiment, the datum is positioned at any locationbetween two sensed cardiac complexes.

Once the datum has been positioned relative the predetermined alignmentfeature, a specified interval is measured between the predeterminedalignment feature and the datum. Two or more morphological features arethen selected along the first model cardiac complex. A measurementinterval is then measured between the datum and each of themorphological features on the first model cardiac complex. In additionto measuring the measurement intervals, the voltage value of the firstmodel cardiac complex at each of the measurement intervals is measuredfrom the first model cardiac signal. The values and locations of thepredetermined alignment feature, the specified interval and themeasurement intervals are then recorded and stored for use inclassifying cardiac complexes as either VT or SVT cardiac complexesduring a tachycardia episode.

When a tachycardia episode is detected, a first cardiac complex and asecond cardiac complex of a cardiac cycle are detected, or sensed, inthe first cardiac signal and the second cardiac signal, respectively. Asthe second cardiac complex is sensed, the predetermined alignmentfeature is identified. The predetermined alignment feature is then usedto define, or position, the datum at the specified interval from thepredetermined alignment feature. Once the datum has been positionedrelative the predetermined alignment feature, a voltage value ismeasured at each of the two or more measurement intervals from the datumalong the first cardiac signal. The voltage values measured from thefirst cardiac signal are then compared voltage values measured frommodel complexes. Based on the comparison, the first cardiac complex isclassified as either a VT complex or a SVT complex.

As the tachycardiac episode occurs, a plurality of cardiac cycles aredetected in the first cardiac signal and the second cardiac signal. Apredetermined number of the first cardiac complexes are classified aseither VT or SVT cardiac complexes based on the present subject matter.A ventricular tachycardia episode is then declared when a thresholdnumber of the predetermined number of the first cardiac complexes areclassified as ventricular tachycardia complexes. Alternatively, asupraventricular tachycardia episode is declared when the thresholdnumber of the predetermined number of the first cardiac complexes areclassified as supraventricular tachycardia complexes.

In an additional embodiment, the present subject matter provides for asystem and method of creating a template for a morphology-basedalgorithm which is used to classify cardiac complexes during atachycardia episode. In one embodiment, electrical energy pulses areprovided to the supraventricular region of the patient's heart. Theresulting cardiac complexes are then sensed and used to create atemplate for use in a morphology-based cardiac classification algorithmfor classifying, categorizing or assessing a patient's cardiaccondition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first cardiac signal and a second cardiac signal whichinclude a first and a second cardiac complex, respectively;

FIG. 2 is a flow diagram of one embodiment of the present subjectmatter;

FIG. 3 is a flow diagram of one embodiment of the present subjectmatter;

FIG. 4 is a flow diagram of one embodiment of the present subjectmatter;

FIG. 5 shows a first cardiac signal and a second cardiac signalaccording to one embodiment of the present subject matter;

FIG. 6 is a flow diagram of one embodiment of the present subjectmatter;

FIG. 7 is a flow diagram of one embodiment of the present subjectmatter;

FIG. 8 is a flow diagram of one embodiment of the present subjectmatter;

FIG. 9 shows a first cardiac signal and a second cardiac signalaccording to one embodiment of the present subject matter;

FIG. 10 is a flow diagram of one embodiment of the present subjectmatter;

FIG. 11 shows a first cardiac signal and a second cardiac signalaccording to one embodiment of the present subject matter;

FIG. 12 is a flow diagram of one embodiment of the present subjectmatter;

FIG. 13 is a flow diagram of one embodiment of the present subjectmatter;

FIG. 14 shows first cardiac signals and second cardiac signals accordingto one embodiment of the present subject matter;

FIG. 15 is a schematic view of one embodiment of an implantable medicaldevice with an endocardial lead and a medical device programmer;

FIG. 16 is a schematic view of one embodiment of an implantable medicaldevice with an endocardial lead and a medical device programmer;

FIG. 17 is a block diagram of an implantable medical device according toone embodiment of the present system; and

FIG. 18 is a block diagram of an implantable medical device according toone embodiment of the present system.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice and use the invention, andit is to be understood that other embodiments may be utilized and thatelectrical, logical, and structural changes may be made withoutdeparting from the spirit and scope of the present invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense and the scope of the present invention is defined by theappended claims and their equivalents.

The present subject matter allows for a cardiac complex to be quicklyand accurately classified as either being an arrhythmic cardiac complexor a non-arrhythmic cardiac complex. One application of the presentsubject matter is in classifying cardiac complexes sensed during atachycardia episode as either ventricular tachycardia (VT) orsupraventricular tachycardia (SVT) complexes. Based on theclassification of the cardiac complexes, the tachycardia episode canthen be classified as either a VT or an SVT event. In one embodiment,the present subject matter is useful in classifying a tachycardiaepisode having a 1:1 atrial-to-ventricular ratio.

In one embodiment the present subject matter uses cardiac signals sensedin two cardiac channels to classify cardiac complexes during atachycardia episode. Using two cardiac channels allows for a morecomplete “view” of each cardiac complex from which to classify thecardiac complex. For example, two cardiac channels useful for thepresent subject matter include a ventricular far-field channel and aventricular near-field channel. FIG. 1 shows one example of a cardiaccomplex as it was sensed in a far-field channel 100 and a near-fieldchannel 110. The cardiac signals are plotted as a voltage of the cardiacsignal (y-axis) as a function of time (x-axis). In the far-field channel100, the cardiac complexes are sensed between defibrillation electrodeswhich provides a signal representative of a larger region of contractingcardiac tissue. For example, the far-field channel 100 includes aQRS-complex 120 which represents a ventricular contraction of a cardiaccycle. In the near-field channel 110, the cardiac complex are sensedbetween a pacing electrode and a second electrode. Due to its smallersize the pacing electrode sensing a more localized region of contractingtissue than the far-field signal. Differences in cardiac signals sensedin far-field and near-field signal are then used in the present subjectmatter to classify cardiac complexes during the tachycardia episode.

The relative positions of the QRS-complexes in the far-field and thenear-field channels contain information about the conduction path of thecardiac action potential. SVT complexes use the same conduction paththat cardiac complexes which initiate in the supraventricular regionuse, as the conduction problem in an SVT is present in thesupraventricular region. In contrast, VT complexes do not use the sameconduction path as cardiac complexes which initiate in thesupraventricular region, as the conduction problem is present in theventricular region. Besides SVT complexes originating in the SVT region,other cardiac complexes also arise in the supraventricular region.Examples include normal sinus rhythm (NSR) complexes and cardiaccomplexes which are initiated by pacing pulses to the supraventricularregion. The similarity in cardiac complex origin between SVT cardiaccomplexes and NSR cardiac complexes or paced cardiac complexes, and thedissimilarity between VT cardiac complexes and the NSR cardiac complexesor paced cardiac complexes, is used in the present subject matter tohelp classify a cardiac complex sensed during a tachycardiac episode aseither a SVT complex or a VT complex.

Referring now to FIG. 2, there is shown one embodiment of the presentsubject matter for classifying cardiac complexes sensed during atachyarrhythmia episode. At 200, a first cardiac signal and a secondcardiac signal are sensed. Both the first and second cardiac signalsinclude cardiac complexes which represent the cardiac cycle of theheart. As a cardiac cycle occurs, a first cardiac complex for a cardiaccycle is detected in the first cardiac signal and a second cardiaccomplex for the cardiac cycle is detected in the second cardiac signalat 210. At 220, a datum is positioned relative the first and secondcardiac complexes. In one embodiment, the datum is a reference point orline from which distances (or times) are measured prior to takingvoltage measurements from the first and second cardiac signals.

In one embodiment, the datum is defined, or located, at a specifiedinterval (e.g., an interval of time) from a predetermined alignmentfeature located on the second cardiac complex. In one embodiment, thepredetermined alignment feature is a repeatably identifiable portion ofa sensed cardiac complex detected in the second cardiac signal which canbe used as a reference point to align and/or coordinate the position ofthe first and second cardiac complexes relative the datum. In oneembodiment, the predetermined alignment feature is the maximumdeflection point of the second cardiac signal during a cardiac complex.In an additional embodiment, the predetermined alignment feature is apoint of maximum slew. Alternatively, the predetermined alignmentfeature is fiducial point of the second cardiac complex. Thepredetermined alignment feature, however, can be any repeatablyidentifiable portion of the second cardiac complex which will be presentregardless of the arrhythmia that is occurring.

At 230, once the datum has been defined (or positioned) relative thepredetermined alignment feature, a voltage value of the first cardiacsignal is measured at each of two or more measurement intervals from thedatum. As will be explained full below, the voltage values measured fromthe first cardiac signal are used to create a complex feature vector.The complex feature vector is then compared to a template featurevector, where in one embodiment the template feature vector represents amodel cardiac complex (or complexes) of a patient's heart. Based on thecomparison, the cardiac complex detected in the first and second cardiacsignal is then classified as either a SVT cardiac complex or a VTcardiac complex.

As the cardiac complexes are sensed during the tachycardia event theyare aligned with and compared to the model cardiac complex. Becausecomparing an entire cardiac complex to a template can be time consumingand/or computationally difficult, the present subject matter measuresvoltage values from the sensed cardiac complexes and creates a vector tonumerically represent the complexes. In one embodiment, model cardiaccomplexes are derived from cardiac complexes sensed from the patient.For example, the model cardiac complex (or complexes) is derived fromcardiac complexes sensed during the patient's normal sinus rhythm (NSR).In an additional example, the model cardiac complex (or complexes) isderived from sensed “induced” cardiac complexes which are the result ofa cardiac cycle initiated by electrical pulses being delivered to thesupraventricular region of the heart. Each of the cardiac complexes tobe classified are then represented by a complex feature vector. Thecomplex feature vector is then compared to the template feature vectorand the cardiac complex is classified based on that comparison.

FIG. 3 shows one embodiment of determining the specified interval andthe template feature vector from the sensed model cardiac complexes. At300, a first cardiac signal and a second cardiac signal are sensed. At310, a first model cardiac complex is detected in the first cardiacsignal, and a second model cardiac complex is detected in the secondcardiac signal. In one embodiment, the first and second model cardiaccomplexes are representative of a cardiac cycle. A predeterminedalignment feature is then located relative the second model cardiaccomplex at 320. In one embodiment, the predetermined alignment featureis positioned, or located, by an attending medical personal who isviewing an image of the second cardiac signal and the second modelcardiac complex. In one embodiment, the image is viewed on a medicaldevice programmer which is in communication with an implantable medicaldevice, such as an implantable cardiac defibrillator, which is sensingthe first and second cardiac signals. The predetermined alignmentfeature is then identified and the medical device programmer is used toprogram the implantable medical device as to the location of thepredetermined alignment feature.

Once the predetermined alignment feature is located at 330, thespecified interval between the predetermined alignment feature and thedatum is measured. In one embodiment, the datum is also positioned, orlocated, by the attending medical personal while viewing the image ofthe first and second cardiac signals. At 340, the specified interval isthen stored. In addition to measuring the specified interval at 350,voltage values of the first cardiac signal during the first modelcardiac complex are measured at each of the two or more measurementintervals from the datum. At 360, the voltage values of the firstcardiac signal at each of the two or more measurement intervals from thedatum are then stored.

As previously discussed, the first and second model cardiac complexescan either be sensed during the patient's normal sinus rhythm (NSR) orbe induced cardiac complexes sensed after electrical pulses have beendelivered to the supraventricular region of the heart. FIG. 4 shows oneexample of determining the specified interval and the template featurevector from NSR cardiac complexes. At 400, a first cardiac signal and asecond cardiac signal are sensed during the patient's NSR. At 410, firstand second model cardiac complexes, in this case NSR cardiac complexes,are detected in the first and second cardiac signals, respectively. Inone embodiment, the NSR cardiac complexes from which the specifiedinterval and the template feature vector are derived can either be asignal NSR complex which is representative of a large number of NSRcomplexes sensed from the patient. Alternatively, an average or medianNSR cardiac complex from two or more NSR cardiac complexes is used todetermine the specified interval and the template feature vector. Toensure that the NSR cardiac complex used in determining an average or amedian NSR cardiac complex are representative of the patient's NSR, acorrelation coefficient for the NSR cardiac complex is calculated andNSR cardiac complexes having a correlation coefficient of greater then0.90 are used to create the representative NSR cardiac complex.

In one embodiment, the patient's NSR are sensed using catheterelectrodes coupled to an implantable cardioverter/defibrillator. In oneembodiment, the NSR complexes are sensed on far and near field signalsusing intracardiac electrodes implanted within the chambers of and/or onthe surface of the patient's heart. The sensed NSR cardiac complexes arethen downloaded, or transferred, to a medical device programmer. In oneembodiment, the medical device programmer displays NSR complexes forreview and selection by attending physicians. The medical deviceprogrammer can also calculate the correlation coefficient for the NSRcomplexes based on morphological features of the complexes and derived amedian or an average NSR complex.

Once the first and second NSR cardiac complexes have been determined,the predetermined alignment feature is identified in the second NSRcardiac complex. In one embodiment, the predetermined alignment featureis identified by the attending physician or medical personal based onthe criteria previously discussed. In one embodiment, the first andsecond NSR cardiac complexes are displayed on the view screen of themedical device programmer. The physician or medical personnel can thenidentify the predetermined alignment feature on the second NSR cardiaccomplex. Alternatively, the predetermined alignment feature isdetermined using an alignment feature extraction program executed in themedical device programmer, where the attending physician or medicalpersonal select the desired predetermined alignment feature from apredetermined list of features the medical device programmer is capableof identifying. The program then analyzes the NSR cardiac complexes toidentify the predetermined alignment feature.

Once the predetermined alignment feature of the second cardiac complexis identified, the value of the specified interval is measured betweenthe predetermined alignment feature and the datum. In one embodiment,the datum is set at a position along the first and second cardiacsignals between two consecutively sensed cardiac complexes. Thespecified interval is then measured between the predetermined alignmentfeature and the datum and the value is then stored. Alternatively, thedatum is set at a specified interval of time from the predeterminedalignment feature, where the specified interval of time is selected soas to position the datum along the first and second cardiac signalsbetween consecutively sensed cardiac complexes. Once the datum is setrelative the predetermined alignment feature the value of the specifiedinterval is stored for use in classifying cardiac complexes.

At 430, voltage values of the first cardiac signal are measured from thefirst NSR cardiac complex relative the datum. In one embodiment, thevoltage values are measured at morphological features of the first NSR.The morphological features selected from the NSR cardiac complexesinclude maximum or minimum deflection points of the cardiac signal, thebeginning or ending of the cardiac complex, fiducial points along thecardiac signals during the NSR cardiac complex. Other selectedmorphological features are also possible, where the features representrepeatably identifiable potion of the first cardiac complex. Thedistance between each of the selected morphology features and the datumis then measured and the value of each of the distances creates ameasurement interval. Each of the measurement intervals is then storedfor subsequent use in locating a portion of the first cardiac signalsensed during a tachycardia episode. A voltage measurement of the firstcardiac signal is then made at each of the measurement interval.

In one embodiment, the voltage values measured at 430 are used to createa template feature vector (TFV). In one embodiment, the TFV=[t₁, t₂, t₃,t₄, . . . ,t_(n)], where the elements of the feature vector (t₁-t_(n))are the voltage value measured from the first cardiac signal at each ofthe two or more measurement intervals from the datum. Once the TFV hasbeen created, the TFV is stored for subsequent use in analyzing cardiaccomplexes during a tachycardiac event.

In one embodiment, the TFV allows the entire waveform of the first NSRcardiac complex to be represented by a series, or vector, of voltagevalues measured along the NSR cardiac complex. By representing the firstNSR cardiac complex with the TFV, the amount of information needed tostore the representation of the complex is greatly reduced compared tostoring the entire signal for the NSR cardiac complex. In addition,since the NSR cardiac complex is being represented by a series of valuesderived from a cardiac signal the computational requirements incomparing the NSR cardiac complex to a cardiac complex sensed during atachycardia episode are also greatly reduced as compared to having toanalyze the morphology of the two cardiac complexes.

Referring now to FIG. 5, there is shown one embodiment of a NSR cardiaccomplex 500 from which a TFV and a specified interval are derived. Afirst cardiac signal 504 is shown having a first NSR cardiac complex 508and a second cardiac signal 512 is shown having a second NSR cardiaccomplex 516. In the present embodiment, the first cardiac signal 504 isa far-field signal and the second cardiac signal 512 is a near-fieldsignal. Other combinations of signals could be used.

A predetermined alignment feature is located along the second NSRcardiac complex 516 as previously discussed. In the present embodiment,the predetermined alignment feature is a maximum deflection point 520 ofthe second NSR cardiac complex 516. As previously discussed, otherpredetermined alignment features along the second NSR cardiac complex516 could be selected. A datum 524 is then positioned as previouslydiscussed at a specified interval 528 from the predetermined alignmentfeature.

At least two morphology features are selected on the first NSR cardiaccomplex 508. In the present embodiment, a plurality of morphologyfeatures are shown at 532. As previously discussed, morphology featurescan be selected at any number of positions along the first NSR cardiaccomplex 508. A measurement interval 540 is then measured between each ofthe selected morphology features 532 and the datum 524. The measurementintervals create the template feature vector as previously described.Additionally, each of the measurement intervals 540 is stored (e.g., thelength of the measurement interval 540 are stored) so subsequent voltagemeasurements can be made along the first cardiac signal at, orapproximately at, the same location relative the predetermined alignmentfeature and the datum during a tachycardiac event.

The present subject matter recognizes that creating a template is notlimited to data derived from a patient's intrinsic NSR cardiaccomplexes. It is also possible to provide electrical energy pulses tothe supraventricular region of the patient's heart from which theresulting induced cardiac complexes are sensed and used to create atemplate for use in a morphology-based cardiac classification algorithmfor classifying, categorizing or assessing a patient's cardiaccondition. FIG. 6 shows an embodiment of creating a template for amorphology-based algorithm which is used to classify cardiac complexesduring a tachyarrhythmia episode. In the present embodiment, electricalenergy pulses are delivered to the supraventricular region of the heartand the resulting cardiac complexes are sensed and utilized in creatingone or more templates for use in a morphology-based classificationalgorithm. As used herein, cardiac complexes started by the delivery ofelectrical energy pulses are referred to as “induced cardiac complexes.”

At 600, at least a first cardiac signal is sensed from the patient'sheart. By way of example only, the first cardiac signal is a far-fieldsignal as previously discussed. At 610, electrical pulses are deliveredto a supraventricular location of the patient's heart. In oneembodiment, the electrical pulses are pacing level pulses deliveredthrough a pacing electrode positioned on or within the supraventricularregion of the patient's heart. The electrical pulses cause the patient'sheart to proceed through a cardiac cycle which is detected as cardiaccomplexes in the first cardiac signal. In one embodiment, the patient isin either NSR and/or in a non-arrhythmic cardiac state. The electricalpulses are delivered to the supraventricular region, such as a rightatrial location adjacent or near to the SA-node. The electrical pulsescan be delivered either at the patient's intrinsic heart rate or at arate that is above the intrinsic cardiac rate. At 620, the first cardiacsignal is detected as the electrical pulses are delivered to thesupraventricular location. In one embodiment, the detected first cardiacsignal includes the model cardiac complexes, which in this case are theinduced cardiac complexes. At 630, the detected, or sensed, inducedcardiac complexes are then used to create the template or“representation” of the patient's non-arrhythmic cardiac complexes.

In one embodiment, creating the template or representation of thepatient's non-arrhythmic cardiac complexes from the induced cardiaccomplexes is accomplished according to the present subject matter. In analternative embodiment, creating the template or representation of thepatient's non-arrhythmic cardiac complexes from the induced cardiaccomplexes is accomplished according to techniques and methods known forcreating templates from non-induced cardiac complexes (e.g., cardiaccomplexes sensed during NSR). In many cases, these non-induced cardiaccomplexes are used to create “templates” which are subsequently used inclassifying, categorizing or assessing sensed cardiac complexes duringan arrhythmic episode.

FIG. 7 shows one embodiment of developing, or creating, a template of aclassification algorithm from detected cardiac complexes from inducedcardiac complexes, where the classification algorithm classifiessubsequently detected cardiac complexes. At 700, an atrial cardiacsignal is sensed from a supraventricular location, where the atrialcardiac signal includes atrial cardiac complexes. In one embodiment, theatrial cardiac signal is sensed with at least one implantable electrodepositioned within a supraventricular location. At 710, an intrinsicatrial rate is calculated from the atrial cardiac complexes. In oneembodiment, the intrinsic atrial rate is an average intrinsic atrialrate.

At 720, electrical pulses are delivered to the supraventricular locationof the patient's heart at a rate that is equal to or greater than thepatient's intrinsic atrial rate. In one embodiment, the electricalpulses are delivered to the supraventricular location at a predeterminedrate which is faster than the average intrinsic atrial rate. In oneembodiment, the predetermined rate is set at a predetermined percentageabove the average intrinsic atrial rate, where the predeterminedpercentage is set in a range of five (5) to fifty (50), five (5) toforty (40) or ten (10) to twenty (20), where ten (10) percent is anacceptable value. In an alternative embodiment, the electrical pulsesare delivered to the supraventricular location at a predetermined targetheart rate. In one embodiment, the predetermined target heart rate isprogrammed in a range of a patient's intrinsic heart rate to one hundredsixty five (165) pulses/minute (or beats/minute), where the patient'sintrinsic heart rate is in the range of forty (40) to eighty (80)beats/minute. Alternatively, the predetermined target heart rate is aprogrammable value in the range of forty (40) to one hundred sixty five(165), eighty (80) to one hundred sixty five (165), forty (40) to onehundred twenty (120), or eighty (80) to one hundred twenty (120)pulses/minute, where seventy (70) pulses/minute is an acceptable value.

In addition, increasing the intrinsic heart rate by deliveringelectrical pulses is accomplished by ramping-up, or increasing, theheart rate at a predetermined ramp acceleration. In one embodiment, thepredetermined ramp acceleration is used to safely accelerate the heartrate from the intrinsic rate to the new induced heart rate (e.g., thepredetermined target heart rate). In one embodiment, the predeterminedramp acceleration is programmed to increase successive cardiac intervalsat no more than five percent of the preceding cardiac interval. In anadditional embodiment, the electrical pulses are pacing level pulsesthat are delivered through a pacing electrode positioned on or withinthe supraventricular region of the patient's heart. In one embodiment,the electrical pacing pulses are programmable voltage values in therange of 0.1 to 10, 0.1 to 5, 1 to 10, or 1 to 5 volts, where 1 volt isan acceptable value.

At 730, the first cardiac signal and the second cardiac signal aresensed from the patient's heart. At 740, first and second model cardiaccomplexes are detected in the first and second cardiac signals as theelectrical pulses are delivered. In one embodiment, the cardiaccomplexes from which the specified interval and the template featurevector are derived can either be a signal cardiac complex which isrepresentative of a large number of the induced cardiac complexes.Alternatively, an average or median cardiac complex from two or more ofthe induced cardiac complexes is used to determine the specifiedinterval and the template feature vector. To ensure that the cardiaccomplex used in determining an average or a median cardiac complex arerepresentative of the patient's induced cardiac complexes, a correlationcoefficient for the cardiac complex is calculated and cardiac complexeshaving a correlation coefficient of greater then 0.90 are used to createthe representative induced cardiac complex.

In one embodiment, the induced cardiac complexes are sensed usingcatheter electrodes coupled to an implantablecardioverter/defibrillator. In one embodiment, the induced cardiaccomplexes are sensed on far-field and near-field signals usingintracardiac electrodes implanted within the chambers of and/or on thesurface of the patient's heart. The sensed induced cardiac complexes arethen downloaded, or transferred, to a medical device programmer. In oneembodiment, the medical device programmer displays induced cardiaccomplexes for review and selection by an attending physician. Themedical device programmer can also calculate the correlation coefficientfor the induced cardiac complexes based on morphological features of thecomplexes and derived a median or an average induced cardiac complexes.

Once the first and second induced cardiac complexes have beendetermined, the predetermined alignment feature is identified in thesecond induced cardiac complex. In one embodiment, the predeterminedalignment feature is identified by the attending physician or medicalpersonal based on the criteria previously discussed. In one embodiment,the first and second induced cardiac complexes are displayed on the viewscreen of the medical device programmer. The physician or medicalpersonnel can then identify the predetermined alignment feature on thesecond induced cardiac complex. Alternatively, the predeterminedalignment feature is determined using an alignment feature extractionprogram executed in the medical device programmer, where the attendingphysician or medical personal select the desired predetermined alignmentfeature from a predetermined list of features the medical deviceprogrammer is capable of identifying. The program then analyzes theinduced cardiac complexes to identify the predetermined alignmentfeature.

At 750, once the predetermined alignment feature of the second inducedcardiac complex is identified the value of the specified interval ismeasured between the predetermined alignment feature and the datum. Inone embodiment, the datum is set at a position along the first andsecond cardiac signals between two consecutively sensed cardiaccomplexes. The specified interval is then measured between thepredetermined alignment feature and the datum and the value stored.Alternatively, the datum is set at a specified interval of time from thepredetermined alignment feature, where the specified interval of time isselected so as to position the datum along the first and second cardiacsignals between consecutively sensed cardiac complexes. Once the datumis set relative the predetermined alignment feature the value of thespecified interval is stored for use in classifying cardiac complexes.

At 760, voltage values of the first cardiac signal are measured from thefirst induced cardiac complex relative the datum. In one embodiment, thevoltage values are measured at morphological features of the firstinduced cardiac complex. The morphological features selected from theinduced cardiac complexes include maximum or minimum deflection pointsof the cardiac signal, the beginning or ending of the cardiac complex,fiducial points along the cardiac signals during the induced cardiaccomplex. Other selected morphological features are also possible, wherethe features represent repeatably identifiable potion of the firstinduced cardiac complex. The distance between each of the selectedmorphology features and the datum is then measured and the value of eachof the distances creates a measurement interval. At 770, each of themeasurement intervals is then stored for subsequent use in locating aportion of the first cardiac signal sensed during a tachycardia episode.A voltage measurement of the first cardiac signal is then made of eachof the measurement interval.

In one embodiment, the voltage values measured at 760 are used to createthe template feature vector (TFV). In one embodiment, the TFV=[t₁, t₂,t₃, t₄, . . . ,t_(n)], where the elements of the feature vector(t₁-t_(n)) are the voltage value measured from the first induced cardiacsignal at each of the two or more measurement intervals from the datum.Once the TFV has been created, the TFV is stored for subsequent use inanalyzing cardiac complexes during a tachycardiac event.

In one embodiment, the TFV allows the entire waveform of the firstinduced cardiac complex to be represented by a series, or vector, ofvoltage values measured along the induced cardiac complex. Byrepresenting the first induced cardiac complex with the TFV, the amountof information needed to store the representation of the complex isgreatly reduced compared to storing the entire signal for the inducedcardiac complex. In addition, since the induced cardiac complex is beingrepresented by a series of values derived from a cardiac signal thecomputational requirements in comparing the induced cardiac complex(i.e., the model cardiac complex) to a cardiac complex sensed during atachycardia episode are also greatly reduced as compared to having toanalyze the morphology of the two cardiac complexes.

Referring now to FIG. 8, there is shown one embodiment of classifying acardiac complex sensed during a tachycardia episode. At 800, a firstcardiac signal and a second cardiac signal are sensed. At 810, the firstand second cardiac signals are analyzed to determine whether atachycardia episode is occurring. In one embodiment, a tachycardiaepisode is detected when a sensed ventricular rate exceeds apredetermined threshold. In one embodiment, the predetermined thresholdis set between 150 to 180 beats per minute. When the ventricular ratedoes not exceed the predetermined threshold, path 814 is taken back to810 and the ventricular rate is analyzed again to determine if atachycardiac event is occurring. Alternatively, when the ventricularrate exceeds the predetermined threshold, path 818 is taken to 820.

At 820, a first cardiac complex and a second cardiac complex are of asensed cardiac cycle detected in the first and second cardiac signals,respectively. As each cardiac complex is sensed, voltage measurementsare made from the cardiac signals. The voltage measurements are thenused to create a complex feature vector (CFV) for each sensed cardiaccomplex. The CFV is then compared to the TFV to classify each of thesensed cardiac complexes as either SVT complexes or VT complexes.

At 830, the second cardiac complex is analyzed to locate thepredetermined alignment feature. As previously discussed, thepredetermined alignment feature of the second cardiac complex is thesame feature located in the second model cardiac complex. In addition tolocating the predetermined alignment feature, the datum is defined atthe specified interval from the predetermined alignment feature on thesecond cardiac complex.

FIG. 9 shows an example of a cardiac complex sensed during a tachycardiaepisode. The first cardiac signal and the second cardiac are shown at900 and 904, respectively. The first and second cardiac signals show afirst cardiac complex 908 and a second cardiac complex 912 whichrepresent a portion of a cardiac cycle sensed during a tachycardiaepisode. In the present embodiment, the predetermined alignment featureis a maximum deflection point 918 in the second cardiac complex, whichwas the predetermined alignment feature used in establishing thespecified interval and the TFV from the NSR cardiac complex. Thespecified interval 528 is then used to define the position of the datum524.

In an additional embodiment, the datum is defined at a scalingpercentage of the specified interval from the predetermined alignmentfeature. A reason for scaling the specified interval is that during atachycardia episode the cardiac complexes occur more rapidly. As aresult the time between subsequent cardiac complexes is shorter thanduring either NSR or the rate of the induced cardiac complexes. Thus,the specified interval may need to be reduced by the scaling percentage,where the specified interval is multiplied by the scaling percentage togive a revised specified interval. In addition to scaling the specifiedinterval, the measurement intervals are also scaled in the same manneras the specified interval. In one embodiment, each of the two or moremeasurement intervals is multiplied by the scaling percentage to giverevised measurement intervals. The voltage value of the first cardiacsignal for the first cardiac complex are then measured at each of two ormore revised measurement intervals from the datum. In one embodiment,the scaling percentage is a function of the sensed ventricular rate,where the scaling percentage decreases as the ventricular rateincreases, where the scaling percentage is programmed at a value between50 and 100 percent.

Referring again to FIG. 8, at 840 voltage values of the first cardiacsignal are measured at the measurement intervals relative the datum. Aspreviously discussed, the measurement intervals for each of the elementsof the TFV were recorded and stored for use in analyzing a tachycardiaccomplex during a tachycardia episode. Unlike having to identifymorphological features to create the measurement intervals in the firstmodel complex measuring the voltage values from the first cardiaccomplex does not rely on identifying the same morphological featuresused to create the measurement intervals. In analyzing and classifyingthe cardiac complex, the measurement intervals are used to measure thedistance, or the interval, from the datum to the points, or areas, atwhich voltage measurements are made from the first cardiac signal.Therefore, one advantage of the present subject matter is thatmorphological features on the first cardiac complex sensed during atachycardia episode do not need to be identified prior to making voltagemeasurements from the first cardiac signal.

Referring again to FIG. 9, there is shown one embodiment of measuringvoltage values from the first cardiac signal 900 at the measurementintervals. By way of example, the measurement intervals are taken as themeasurement intervals 540 defined in FIG. 5. In FIG. 5, five voltagemeasurements were made at five different measurement intervals 540 whichgave a five element TFV=[t₁, t₂, t₃, t₄, t₅]. The same measurementintervals 540 are used to measure the distance from the datum 524 atwhich the voltage measurements are to be made along the first cardiacsignal 900. The voltage values are then used to create the cardiacfeature vector (CFV), CFV=[c₁, c₂, c₃, c₄, . . . ,c_(n)], where theelements of the cardiac feature vector (c₁-c_(n)) are the voltage valuemeasured from the first cardiac signal at each of the two or moremeasurement intervals from the datum. In the present embodiment becausethe same number of measurement intervals are used to measure voltagevalues from the first cardiac signal as were measured from the firstmodel cardiac complex (e.g., NSR cardiac complex or induced cardiaccomplex), a five element CFV is created CFV=[c₁, c₂, c₃, c₄, c₅].

Referring again to FIG. 8, once the voltage values of the first cardiacsignal have been measured at the measurement intervals from the datum,the voltage values of the first cardiac complex and the model cardiaccomplex are compared to determine whether the cardiac complex is an SVTcardiac complex or a VT cardiac complex. At 850, the comparison betweenthe two cardiac complexes is accomplished using the CFV of the cardiaccomplex and the TFV of the model cardiac complex. One example ofcomparing the CFV and the TFV is to calculate a correlation coefficient,r, of the CFV and the TFV as follows:

r=correcoef(TFV,CFV)

where correcoef (TFV, CFV) is the correlation coefficient betweenvectors TFV and CFV. A value of +1.0 means that TFV and CFV arecorrelated. As the correlation coefficient, r, value falls below 1.0 thecardiac complex becomes less correlated with the model cardiac complex.

At 860, the correlation coefficient computed for the TFV and the CFV forthe cardiac complex is then compared to a predetermined threshold, β.When the correlation coefficient is greater than the predeterminedthreshold, the cardiac complex is classified as a SVT cardiac complex at870. When the correlation coefficient is less than or equal to thepredetermined threshold, the cardiac complex is classified as a VTcardiac complex at 880.

In one embodiment, once a tachycardiac episode is detected, a pluralityof cardiac cycles are sensed and classified according to the presentsubject matter. The classified cardiac complexes are then used toclassify the tachycardia episode as either a ventricular tachycardiaepisode or a supraventricular tachycardia episode. An example ofclassifying the tachycardia episode include using an X out of Ycriterion, where Y is a predetermined number of cardiac complexes (e.g.,first cardiac complexes) which are sensed and classified. In oneembodiment, Y defines a window of consecutively sensed cardiaccomplexes, where the window has a predetermined number of sensed cardiaccomplexes and a new window occurs with each consecutive cardiac complex.When a threshold number of the first cardiac complexes are classified aseither VT or SVT complexes during a window of Y cardiac complexes thetachycardia episode is classified (or declared) as either being aventricular tachycardia episode or a supraventricular tachycardiaepisode. In one embodiment, Y is a programmable value in the range of 8to 50, where 10 is an acceptable number of cardiac complexes. X is alsoa programmable value in the range of 5 to 10, where 7 is an acceptablenumber when the number 10 is programmed for X.

In an alternative embodiment, a percentage threshold of the classifiedcardiac complexes is used to classify the tachycardia episode. Forexample, after classifying the cardiac complex a percentage of theclassified cardiac complexes is calculated for the plurality of sensedcardiac cycles. The calculated percentage of VT complexes and SVTcomplexes sensed during the tachycardia episode is compared to thepercentage threshold. So, a ventricular tachycardia episode is declaredwhen the threshold percentage of the first cardiac complexes from theplurality of cardiac cycles are classified as ventricular tachycardiacomplexes. Alternatively, a supraventricular tachycardia episode isdeclared when the threshold percentage of the first cardiac complexesfrom the plurality of cardiac cycles are classified as supraventriculartachycardia complexes. In one embodiment, the predetermined percentagethreshold is a programmable value in the range of 50 to 100 percent,where a value of approximately 70 percent is an acceptable value. Theplurality of cardiac complexes used in calculating the percentage of VTand SVT complexes is also a programmable number, where the plurality ofcardiac complexes is programmed in the range of between 8 to 50, where10 is an acceptable value.

Referring now to FIG. 10, there is shown an additional embodiment of thepresent subject matter. At 1000 a first cardiac signal and a secondcardiac signal of a sensed cardiac complex are sensed. At 1010, firstand second model cardiac complexes (e.g., NSR cardiac complexes orinduced cardiac complexes) are detected in the first and second cardiacsignals, respectively, as previously described. The first and secondmodel cardiac complexes are then positioned in an analysis window at1020. In one embodiment, the analysis window is a defined area around aportion of the first and second cardiac signals.

Referring now to FIG. 11, there is shown one embodiment of first andsecond cardiac signals, 1100 and 1104, positioned within an analysiswindow 1108. The analysis window 1108 is positioned around a first modelcardiac complex 1112 and a second model cardiac complex 1116. In oneembodiment, at least a portion of the analysis window 1108 is used asthe datum for measuring the specified interval from the predeterminedalignment feature on the second cardiac complex 1116. In one embodiment,the datum is selected from any position along a first horizontal axis1120 of the analysis window 1108. For example, the datum could beselected from a vertical line 1124 positioned at the time the analysiswindow 1108 begins. Alternatively, the datum could be selected from avertical line 1128 positioned at the time at which the analysis window1108 ends.

Referring again to FIG. 10, once the analysis window has been positionedaround the first and second model cardiac complex a predeterminedalignment feature is selected, or identified, from the second modelcardiac complex. Once the predetermined alignment feature of the secondcardiac complex is identified, the specified interval is measuredbetween the predetermined alignment feature and the datum at 1030. Oncethe datum is set relative the predetermined alignment feature themeasurement interval is stored for use in classifying cardiac complexes.Voltage values of the first cardiac signal are then measured at themeasurement intervals on the first model cardiac complex as previouslydescribed.

At 1150, voltage values of the first cardiac signal are measured fromthe first model cardiac complex relative the datum as previouslydescribed. The distance between each of the selected morphology featuresand the datum is then measured and the value of each of the distancescreates a measurement interval. Each of the measurement intervals isthen stored for subsequent use in locating a portion of the firstcardiac signal sensed during a tachycardia episode. A voltagemeasurement of the first cardiac signal is then made at each of themeasurement intervals and the TFV for the model cardiac complex iscreated and stored for subsequent use in analyzing cardiac complexesduring a tachycardiac event.

In FIG. 12 there is shown one embodiment of classifying a cardiaccomplex according to the present subject matter. The encircled number 1in FIG. 10 provides a link to the encircled number 1 in FIG. 12. At1200, a first and second cardiac complex of a sensed cardiac complex issensed in the first and second cardiac signals, respectively, during atachycardia episode. At 1210, the first and second cardiac complex arepositioned within an analysis window. The alignment feature is thenlocated on the second cardiac complex. In one embodiment, the analysiswindow has the same horizontal axis dimension, or length, of theanalysis window used for the first and second model cardiac complex increating the TFV. The analysis window is then positioned around thefirst and second cardiac complex so the alignment feature is located atthe specified interval from the datum.

Voltage values are then measured from the first cardiac signal at themeasurement intervals at 1220. The voltage values for the first cardiaccomplex are then compared to the voltage values for the first modelcardiac complex to determine whether the first cardiac complex is a VTcomplex or a SVT complex. In one embodiment, the comparison isaccomplished by creating a CFV for the cardiac complex. The CFV and theTFV are then compared at 1230. In one embodiment, a correlationcoefficient, r, is calculated for the CFV and the TFV. The correlationcoefficient computed for the TFV and the CFV for the cardiac complex isthen compared to the predetermined threshold, β, at 1240. When thecorrelation coefficient is greater than the predetermined threshold, thecardiac complex is classified as a SVT cardiac complex at 1250. When thecorrelation coefficient is less than or equal to the predeterminedthreshold, the cardiac complex is classified as a VT cardiac complex at1260.

Referring now to FIG. 13, there is shown an additional embodiment of thepresent subject matter. The encircled number 1 in FIG. 10 provides alink to the encircled number 1 in FIG. 13. At 1300, a first and secondcardiac complex is sensed in the first and second cardiac signals,respectively, during a tachycardia episode. At 1310, the first andsecond cardiac complex are positioned relative the first and secondmodel cardiac complexes within an analysis window. In one embodiment,the alignment feature of the second cardiac complex is located and thenaligned with the alignment feature on the second model cardiac complex.In the present embodiment, the analysis window has the same horizontalaxis dimension, or length, of the analysis window used for the first andsecond model cardiac complex in creating the TFV. The analysis window isthen positioned around the first and second cardiac complex and thefirst and second model cardiac complex so the alignment feature on boththe second model cardiac complex and the second cardiac complex arelocated at the measurement interval from the datum.

FIG. 14 shows one example of both the first and second cardiac complexand the first and second model cardiac complex aligned within ananalysis window 1400. A first model cardiac complex 1406 and a secondmodel cardiac complex 1412 are positioned within the analysis window1400 with an alignment feature 1418 on the second model cardiac complex1412 positioned at a specified interval 1424 from a datum 1428. A firstcardiac complex 1430 and a second cardiac complex 1436 of a cardiaccycle sensed during a tachycardia episode are also positioned within theanalysis window 1400. In one embodiment, a predetermined alignmentfeature 1440 on the second cardiac complex 1436 is aligned with thepredetermined alignment feature 1418 on the second model cardiac complex1412. Alternatively, the predetermined alignment feature 1440 on thesecond cardiac complex 1436 is positioned at the specified interval 1424from the datum 1428.

Voltage values are then measured from the first cardiac signal at themeasurement intervals 1450. The voltage value at each of the measurementintervals 1450 for the first cardiac complex are then compared to thevoltage value in each of the measurement intervals 1450 for the firstmodel cardiac complex to determine whether the first cardiac complex isa VT cardiac complex or a SVT cardiac complex. In one embodiment, thecomparison is accomplished by creating a CFV for the cardiac complex.The CFV and the TFV are then compared at 1330. In one embodiment, acorrelation coefficient, r, is calculated for the CFV and the TFV. Thecorrelation coefficient computed for the TFV and the CFV for the cardiaccomplex is then compared to the predetermined threshold, β, at 1340.When the correlation coefficient is greater than the predeterminedthreshold, the cardiac complex is classified as a SVT cardiac complex at1350. When the correlation coefficient is less than or equal to thepredetermined threshold, the cardiac complex is classified as a VTcardiac complex at 1360.

In one embodiment, the present subject matter can be used in animplantable cardiac defibrillator (ICD) and/or an external medicaldevice programmer. The present subject matter is compatible with ICDsystems having one or more intracardiac leads having one or moreelectrodes which are able to sense a first and second cardiac signals.The present medical system can also be implemented in an externalcardioverter/monitor system which includes surface electrodes and/orintracardiac leads having one or more electrodes. The present subjectmatter can also be implemented in an implantable atrialcardioverter-defibrillator, which may include numerous pacing modesknown in the art. Furthermore, although the present system is describedin conjunction with an implantable cardiac defibrillator having amicroprocessor based architecture, it will be understood that theimplantable cardiac defibrillator (or other implanted device) may beimplemented in any logic based, custom integrated circuit architecture,if desired.

Referring now to FIG. 15, there is shown one embodiment of a system,such as a medical device system, which includes at least one pacingelectrode and at least a first defibrillation electrode and a seconddefibrillation electrode. In one embodiment, the system includes animplantable cardiac defibrillator 1500 electrically and physicallycoupled to at least one intracardiac catheter 1502. In the presentembodiment the intracardiac catheter 1502 includes the at least onepacing electrode and the first and second defibrillation electrodes.Other catheter having either the first defibrillation electrode, thesecond defibrillation electrode or additional defibrillation electrodescould be included in the present system.

The intracardiac catheter 1502 shown in FIG. 15 is an endocardial leadadapted to be releasably coupled to the cardiac defibrillator 1500. Theintracardiac catheter 1502 has an elongate body with a proximal end 1508and a distal end 1510. The intracardiac catheter 1502 includes a pacingelectrode 1512 located at, or adjacent, the distal end 1510 of theintracardiac catheter 1502. Additional pacing electrodes can also beincluded on the intracardiac catheter 1502 to allow for bipolar sensingand pacing with the pacing electrode 1512. In addition, other pacing andsensing electrode configurations are also possible.

The intracardiac catheter 1502 further includes one or moredefibrillation electrodes. The intracardiac catheter 1502 of FIG. 15includes a first defibrillation electrode 1514 and a seconddefibrillation electrode 1516, where the first defibrillation electrode1514 and the second defibrillation electrode 1516 are defibrillationcoil electrodes. The first defibrillation electrode 1514 is spaced apartand proximal from the pacing electrode 1512, and the seconddefibrillation electrode 1516 is spaced apart and proximal from thefirst defibrillation electrode 1514. One example of intracardiaccatheter 1502 is an Endotak catheter (CPI/Guidant, St. Paul, Minn.).

Additional intracardiac catheters can be included with a medical devicesystem. For example, FIG. 16 shows one embodiment of the implantablecardiac defibrillator 1500 having an intracardiac catheter 1502 aspreviously described and a second intracardiac catheter 1604. In oneembodiment, the second intracardiac catheter 1604 includes asupraventricular pacing electrode 1610. In addition to thesupraventricular pacing electrode 1610, the second intracardiac catheter1604 can further include additional pacing/sensing electrodes and/ordefibrillation electrodes as are known. Cardiac signals, such as anatrial cardiac signal, can be sensed from the pacing and defibrillationelectrodes positioned on the second intracardiac catheter and deliveredto the implantable cardiac defibrillator 1500 for analysis andelectrical pulses can be delivered to the supraventricular pacingelectrode 1610 according to the present subject matter.

Referring now to FIG. 17, there is shown an embodiment of a blockdiagram of the system of FIG. 15 which includes the cardiacdefibrillator 1500. The cardiac defibrillator 1500 includes controlsystem circuitry 1700 which is coupled to a sensing system 1702. Thesensing system 1702 includes terminals labeled with reference numbers1704, 1708, and 1712 for connection to electrodes coupled to the surfaceof the intracardiac catheter 1502. The pacing electrode 1512 iselectrically connected to terminal 1704 and to the control systemcircuitry 1700 through an electrically insulated conductor providedwithin the elongate body of the intracardiac catheter 1502. The firstdefibrillation electrode 1514 and the second defibrillation electrode1516 are connected to terminals 1708 and 1712, respectively, and to thecontrol system circuitry 1700 through electrically insulated conductorsprovided within the elongate body of the intracardiac catheter 1502.

In one embodiment, the control system circuitry 1700 is encased andhermetically sealed in a housing 1716 which is suitable for implantingin a human body. A connector block 1720 is additionally attached to thehousing 1716 of the cardiac defibrillator 1500 to allow for the physicaland the electrical attachment of the intracardiac catheter 1502 and theelectrodes to the cardiac defibrillator 1500 and the encased controlsystem circuitry 1700.

In one embodiment, the control system circuitry 1700 of the cardiacdefibrillator 1500 is a programmable microprocessor-based system, with amicroprocessor 1724 and a memory circuit 1726, which contains parametersfor various pacing, defibrillation, and sensing modes and stores dataindicative of cardiac signals received by the control system circuitry1700. A transmitter circuit 1728 is additionally coupled to the controlsystem circuitry 1700 and the memory circuit 1726 to allow the cardiacdefibrillator 1500 to communicate with a medical device programmer 1730.In one embodiment, the transmitter circuit 1728 and the medical deviceprogrammer 1730 use a wire loop antenna 1734 and a radio frequencytelemetric link, as is known in the art, to receive and transmit signalsand data to and from the programmer unit 1730 and the control systemcircuitry 1700. The second cardiac signal is a near-field signal sensedthrough the pacing electrode 1512. In the present embodiment, the pacingelectrode 1512 and the first defibrillation electrode 1514 are used tosense the near-field cardiac signal or rate signal. In one embodiment,the pacing electrode 1512 and first defibrillation electrode 1514 arecoupled to a sense amplifier 1740 within the sensing system 1702 toallow for the second cardiac signal as previously described to besensed. In an alternative embodiment, the connections of the presentcontrol circuit is adapted to allow for the near-field cardiac signalsto be sensed between the pacing electrode 1512 and the housing 1716.

In the present system, the first cardiac signal is a far-field signal.In one embodiment, the first cardiac signal is sensed between the firstdefibrillation electrode 1514 and the second defibrillation electrode1516 which are coupled to the sense amplifier 1755. In an additionalembodiment, the connections of the present control circuitry is adaptedto allow for the first cardiac signal can also be sensed between thefirst defibrillation electrode 1514, the second defibrillation electrode1516 and the housing 1530. The control system 1700 monitors the firstcardiac signal for a first cardiac complex and the second cardiac signalfor a second cardiac complex, where the first cardiac complex and thesecond cardiac complex represent a cardiac cycle.

The output of the sense amplifier 1740 is shown connected to an R-wavedetector 1744 which is coupled to the control system 1700. In oneembodiment, the R-wave detector 1744 determines the ventricular ratefrom the sensed cardiac complexes. The ventricular rate is then suppliedto the microprocessor 1724. In one embodiment, the microprocessor 1724analyzes the ventricular rate to detect when the ventricular rateexceeds a predetermined threshold. When the ventricular rate exceeds thepredetermined threshold, the microprocessor 1724 declares a tachycardiaepisode. In one embodiment, the predetermined threshold is aprogrammable value set between 150 to 180 beats per minute.

When a tachycardia episode is declared, the R-wave detector circuit 1744detects the second cardiac complex in the second cardiac signal andprovides the second cardiac signal to an alignment circuit 1750 coupledto the control system 1700. In one embodiment, the alignment circuit1750 analyzes the second cardiac complex to locate a predeterminedalignment feature on the second cardiac complex and positions a datum ata specified interval from the alignment feature on the second cardiaccomplex.

The alignment circuit 1750 also receives the first cardiac signal anddetects the first cardiac complex as the cardiac cycle is sensed. Thealignment circuit 1750 receives the first cardiac complex and the secondcardiac complex of the sensed cardiac cycle and analyzes the secondcardiac complex to locate the predetermined alignment feature. In oneembodiment, a location algorithm according to the present subject matterfor identifying the alignment feature on the second cardiac complex isstored in the memory 1726 and executed in the alignment circuit 1750.The alignment circuit 1750 also positions the datum at the specifiedinterval from the alignment feature on the second cardiac complex. Inone embodiment, the specified interval of the datum is stored in memory1726. In an alternative embodiment, the alignment circuit 1750 positionsthe analysis window around the first and second cardiac complex aspreviously described when an analysis window is used to isolate acardiac complex in a first and second cardiac signal.

A morphology analyzing circuit 1754 is coupled to the alignment circuit1750 of the control system 1700 via bus 1758. The morphology analyzingcircuit 1754 measures the voltage value of the first cardiac signal ateach of two or more measurement intervals from the datum. In oneembodiment, the morphology analysis circuit 1754 retrieves themeasurement intervals from the memory 1726. Once the first and secondcardiac complex have been aligned relative the datum, the morphologyanalyzing circuit 1754 measures the voltage of the first cardiac signalat each of the measurement intervals from the datum.

A vector comparison circuit 1760 is coupled to the control system 1700,including the morphology analyzing circuit 1754, via bus 1758. Thevector comparison circuit 1760 receives the voltage values measured atthe measurement intervals and creates the complex feature vector foreach of the sensed cardiac complexes. The vector comparison circuit 1760then compares the complex feature vector to the TFV. In one embodiment,the vector comparison circuit 1760 calculates the correlationcoefficient between the TFV and the CFV of each of the sensed cardiaccomplexes. The vector comparison circuit 1760 then compares thecorrelation coefficient to the predetermined threshold, β. In oneembodiment, the predetermined threshold is stored in the memory 1726.The vector comparison circuit 1760 classifies the cardiac complex as aSVT cardiac complex when the correlation coefficient is greater than thepredetermined threshold and classifies the cardiac complex as a VTcardiac complex when the correlation coefficient is less than or equalto the predetermined threshold.

In one embodiment, the sensing system 1702 detects a plurality of thecardiac cycles in the first cardiac signal and the second cardiacsignal. As the plurality of first cardiac complexes are sensed, thevector comparison circuit 1760 classifies a predetermined number of thefirst cardiac complexes. The microprocessor 1724 receives theclassification of the cardiac complexes as they are classified by thevector comparison circuit 1760. In one embodiment, the predeterminednumber of the first cardiac complexes is a window of X cardiaccomplexes, where a new window is created as each subsequent cardiaccomplex is analyzed and classified. When a threshold number, Y, of X ofthe cardiac complexes are classified as VT complexes the control system1700 declares a VT episode. Alternatively, when the Y of X counter failsto detect the threshold number of VT complexes in the X complex window,the ventricular arrhythmia is classified as a SVT episode. In analternative embodiment, as the microprocessor 1724 receives theclassification of the cardiac complexes a percentage of the classifiedcardiac complexes is calculated. The calculated percentage of VTcomplexes and SVT complexes sensed for the plurality of cardiac cyclesduring the tachycardia episode is compared to the threshold number,where the threshold number is a percentage value. So, a ventriculartachycardia episode is declared when the threshold number, orpercentage, of the first cardiac complexes from the plurality of cardiaccycles are classified as ventricular tachycardia complexes.Alternatively, a supraventricular tachycardia episode is declared whenthe threshold number of the first cardiac complexes from the pluralityof cardiac cycles are classified as supraventricular tachycardiacomplexes. In one embodiment, the threshold number is a programmablevalue in the range of 50 to 100 percent, where a value of approximately70 percent is an acceptable value. The plurality of cardiac cyclessensed for classification and used in calculating the percentage of VTand SVT complexes is also a programmable number, where a value in therange of 8 to 50 classified complexes, where 10 is an acceptable value.

In one embodiment, once the ventricular episode has been classified, themicroprocessor generates a signal which is delivered to a therapy outputcircuit 1770. In one embodiment, the therapy output circuit 1770generates electrical energy (e.g., cardioversion and/or defibrillationelectrical energy) which is delivered between the first and seconddefibrillation electrodes. Power for the cardiac defibrillator 1500 issupplied by an electrochemical battery 1774 that is housed within thecardiac defibrillator 1500.

Referring now to FIG. 18, there is shown an embodiment of a blockdiagram of cardiac defibrillator 1500 shown in FIG. 16. The cardiacdefibrillator 1500 of FIG. 18 includes all the components previouslydescribed in FIG. 17. In addition, the sensing system 1702 of thecardiac defibrillator 1500 further includes terminal 1810 for connectionto the supraventricular pacing electrode 1610 which is coupled to thesurface of the second intracardiac catheter 1604. In addition, theconnector block 1720 further includes a second socket which is adaptedto receive the second intracardiac catheter 1604.

The sensing system 1702 is used to detect an atrial cardiac signal withthe supraventricular pacing electrode 1610. The control system 1700monitors the atrial cardiac signal for atrial cardiac complexes andcalculates an intrinsic atrial rate from the detected atrial cardiaccomplexes. In one embodiment, the sensing system 1702 includes an atrialsignal sense amplifier 1814 which receives the atrial cardiac signal.The signal from the atrial signal sense amplifier 1814 is delivered to aP-wave detector circuit 1820 which detects the occurrence of atrialcardiac complexes (atrial contractions) from the atrial cardiac signal.The P-wave detector circuit 1820 is coupled to bus 1758 which allow forthe microprocessor 1724 to calculate an intrinsic atrial rate, includingan average intrinsic atrial rate, from the sensed atrial cardiaccomplexes.

The control system 1700 of FIG. 18 further includes a pace outputcircuit 1824. The pace output circuit 1824 is coupled to the terminal1810 to allow for electrical pulses to be delivered to thesupraventricular pacing electrode at the predetermined rate, which canbe either at the intrinsic atrial rate or at a rate that is faster thanthe average intrinsic atrial rate as previously described. In oneembodiment, the electrical pulses are delivered under the control of thecontrol system 1700.

As previously described, the control system 1700 monitors the firstcardiac signal for the first model cardiac complexes as electricalpulses are delivered to the supraventricular pacing electrode. Themorphology analyzing circuit 1754 receives the detected first modelcardiac complexes for the creation of a template. In one embodiment, thetemplate is a classification algorithm which is used to classifysubsequently detected cardiac complexes. Additionally, the morphologyanalyzing circuit creates a model cardiac complex from the first modelcardiac complexes. Alternatively, the control system 1700 monitors thesecond cardiac signal for second model cardiac complexes as electricalpulses are delivered to the supraventricular pacing electrode. TheR-wave detector circuit 1744 detects the second model cardiac complexesin the second cardiac signal and provides the second cardiac signal tothe alignment circuit 1750. The alignment circuit 1750 analyzes each ofthe second model cardiac complexes to locate the predetermined alignmentfeature on each of the second model cardiac complexes. In addition, thealignment circuit 1750 positions the datum at the specified intervalfrom the alignment feature on each of the second model cardiaccomplexes.

The morphology analyzing circuit 1754 then measures a voltage value ofthe first cardiac signal at each of two or more measurement intervalsfrom the datum for each of the first model cardiac complexes. In oneembodiment, once the morphology analyzing circuit 1754 has measured thevoltage values of the cardiac signals the morphology analyzing circuit1754 creates the template feature vector from the voltage value of thefirst cardiac signal at each of two or more measurement intervals fromthe datum for each of the first model cardiac complexes. In analternative embodiment, the model cardiac complexes detected in thefirst and/or second cardiac complexes are downloaded to the programmer1730. Once downloaded, the programmer 1730 is used to create thetemplate from the model cardiac complexes. The template is thentransmitted back to the control system 1700 where it is stored in memory1726 for use in analyzing and classifying cardiac complexes sensedduring a tachycardia episode.

1. An implantable cardiac defibrillator configured to be coupled to afirst defibrillation electrode, a second defibrillation electrode, asupraventricular pacing electrode, and a further pacing electrode, thedefibrillator comprising: a sensing system configured to be coupled tothe first defibrillation electrode and the second defibrillationelectrode and detect a first cardiac signal between the firstdefibrillation electrode and the second defibrillation electrode; acontrol system coupled to the sensing system and configured to monitorthe first cardiac signal for first cardiac complexes including firstmodel cardiac complexes and first sensed cardiac complexes; a paceoutput circuit coupled to the control system and the supraventricularelectrode and configured to deliver electrical pulses to thesupraventricular pacing electrode under control of the control system,wherein the control system is configured to monitor the first cardiacsignal for the first model cardiac complexes as the electrical pulsesare delivered to the supraventricular pacing electrode; and a morphologyanalyzing circuit coupled to the control system and configured toreceive the first model cardiac complexes for creation of a templatefeature vector for classification of the first sensed cardiac complexes.2. The defibrillator of claim 1, wherein the sensing system isconfigured to be coupled to the further pacing electrode and detect asecond cardiac signal through the further pacing electrode, and thecontrol system is configured to monitor the second cardiac signal forsecond model cardiac complexes as the electrical pulses are delivered tothe supraventricular pacing electrode, and comprising: an R-wavedetector circuit coupled to the control system and configured to detectthe second model cardiac complexes in the second cardiac signal; and analignment circuit coupled to the R-wave detector and configured toanalyze each of the second model cardiac complexes to locate apredetermined alignment feature on each of the second model cardiaccomplexes and positions a datum at a specified interval from thealignment feature on the each of the second model cardiac complexes, andwherein the morphology analyzing circuit is configured to measure avoltage value of the first cardiac signal at each of two or moremeasurement intervals from the datum for each of the first model cardiaccomplexes from which the template feature vector is created from thevoltage value of the first cardiac signal at each of two or moremeasurement intervals from the datum for each of the first model cardiaccomplexes.
 3. The defibrillator of claim 2, wherein the sensing systemis configured to be coupled to the supraventricular pacing electrode anddetect an atrial cardiac signal from the supraventricular pacingelectrode, the control system is configured to monitor the atrialcardiac signal for atrial cardiac complexes and calculate an averageintrinsic atrial rate from the detected atrial cardiac complexes, andthe pace output circuit is configured to deliver the electrical pulsesto the supraventricular pacing electrode at a predetermined rate whichis faster than the average intrinsic atrial rate.
 4. The defibrillatorof claim 2, wherein the control system comprises a memory circuit tostore the template feature vector, and the morphology analyzing circuitis configured to measure a voltage value of the first cardiac signal ateach of two or more measurement intervals from the datum for each firstsensed cardiac complex of the first sensed cardiac complexes from whicha complex feature vector is created from the voltage value of the firstcardiac signal at each of two or more measurement intervals from thedatum for each of the first sensed cardiac complexes, and comprising avector comparison circuit coupled to the control system and configuredto receive the voltage values measured at the measurement intervals,create the complex feature vector, compare the complex feature vector tothe template feature vector, and classify the each first sensed cardiaccomplex using the comparison of the complex feature vector and thetemplate feature vector.
 5. The defibrillator of claim 4, wherein thevector comparison circuit is configured to calculate a correlationcoefficient from the template feature vector and the complex featurevector and classify the each first sensed cardiac complex using thecorrelation coefficient and a threshold.
 6. The system of claim 5,wherein the vector comparison circuit is configured to classify the eachfirst sensed cardiac complex as a ventricular tachycardia complex whenthe correlation coefficient is less than or equal to the threshold, andclassify the each first cardiac complex as a supraventriculartachycardia complex when the correlation coefficient is greater than thethreshold.
 7. The system of claim 6, wherein the vector comparisoncircuit is configured to classify a specified number of the first sensedcardiac complexes, and the control system is configured to declare aventricular tachycardia episode when at least a threshold number of thespecified number of the first sensed cardiac complexes are classified asventricular tachycardia complexes.
 8. A system, comprising: asupraventricular pacing electrode; at least a first defibrillationelectrode and a second defibrillation electrode; a sensing systemcoupled to the supraventricular pacing electrode and the firstdefibrillation electrode and the second defibrillation electrode, thesensing system configured to detect a first cardiac signal between thefirst defibrillation electrode and the second defibrillation electrode;a control system coupled to the sensing system and configured to monitorthe first cardiac signal for first cardiac complexes including firstmodel cardiac complexes and first sensed cardiac complexes; a paceoutput circuit coupled to the control system and configured to deliverelectrical pulses to the supraventricular pacing electrode under controlof the control system, wherein the control system is configured tomonitor the first cardiac signal for the first model cardiac complexesas the electrical pulses are delivered to the supraventricular pacingelectrode; and a morphology analyzing circuit coupled to the controlsystem and configured to receive the first model cardiac complexes forcreation of a template feature vector for classification of the firstsensed cardiac complexes.
 9. The system of claim 8, comprising: afurther pacing electrode coupled to the sensing system, wherein thesensing system is configured to detect a second cardiac signal throughthe further pacing electrode, and the control system is configured tomonitor the second cardiac signal for second model cardiac complexes asthe electrical pulses are delivered to the supraventricular pacingelectrode; an R-wave detector circuit coupled to the control system andconfigured to detect the second model cardiac complexes in the secondcardiac signal; and an alignment circuit coupled to the R-wave detectorand configured to analyze each of the second model cardiac complexes tolocate a predetermined alignment feature on each of the second modelcardiac complexes and positions a datum at a specified interval from thealignment feature on each of the second model cardiac complexes, andwherein the morphology analyzing circuit is configured to measure avoltage value of the first cardiac signal at each of two or moremeasurement intervals from the datum for each of the first model cardiaccomplexes from which the template feature vector is created from thevoltage value of the first cardiac signal at each of two or moremeasurement intervals from the datum for each of the first model cardiaccomplexes.
 10. The system of claim 9, wherein the sensing system isconfigured to detect an atrial cardiac signal from the supraventricularpacing electrode, the control system is configured to monitor the atrialcardiac signal for atrial cardiac complexes and calculate an averageintrinsic atrial rate from the detected atrial cardiac complexes, andthe pace output circuit is configured to deliver the electrical pulsesto the supraventricular pacing electrode at a predetermined rate whichis faster than the average intrinsic atrial rate.
 11. The system ofclaim 9, wherein the control system comprises a memory circuit to storethe template feature vector, and the morphology analyzing circuit isconfigured to measure a voltage value of the first cardiac signal ateach of two or more measurement intervals from the datum for each firstsensed cardiac complex of the first sensed cardiac complexes from whicha complex feature vector is created from the voltage value of the firstcardiac signal at each of two or more measurement intervals from thedatum for each of the first sensed cardiac complexes, and comprising avector comparison circuit coupled to the control system and configuredto receive the voltage values measured at the measurement intervals,create the complex feature vector, compare the complex feature vector tothe template feature vector, and classify the each first sensed cardiaccomplex using the comparison of the complex feature vector and thetemplate feature vector.
 12. The system of claim 11, wherein the vectorcomparison circuit is configured to calculate a correlation coefficientfrom the template feature vector and the complex feature vector andclassify the each first sensed cardiac complex by comparing thecorrelation coefficient to a threshold.
 13. The system of claim 12,wherein the vector comparison circuit is configured to classify aspecified number of the first sensed cardiac complexes, and the controlsystem is configured to declare a ventricular tachycardia episode whenat least a threshold number of the specified number of the first sensedcardiac complexes are classified as ventricular tachycardia complexes.14. The system of claim 9, comprising: a first intracardiac catheterincluding the first defibrillation electrode, the second defibrillationelectrode, and the further pacing electrode; and a second intracardiaccatheter including the supraventricular pacing electrode.
 15. The systemof claim 14, comprising: a medical device programmer; and a cardiacdefibrillator coupled to the first and second intracardiac catheters andincluding an implantable housing encasing the control system and atransmitter/receiver coupled to the control system and configured totransmit signals to and receive signals from the medical deviceprogrammer, wherein the medical device programmer is configured totransmit a first signal to control the alignment circuit to locate thepredetermined alignment feature on the second model cardiac complex andto position the datum at the specified interval from the alignmentfeature on the second model cardiac complex, and transmit a secondsignal to supply the two or more measurement intervals to the morphologyanalyzing circuit.
 16. A method comprising: detecting, at differingfirst and second locations, respective first and second cardiaccomplexes that are both associated with a first heart depolarization;detecting a fiducial point in the first cardiac complex and noting acorresponding first cardiac complex fiducial point time; establishingmeasurement intervals using times corresponding to features in thesecond cardiac complex relative to the first cardiac complex fiducialpoint time; detecting, at the first and second locations, respectivethird and fourth cardiac complexes that are both associated with asecond heart depolarization instance; detecting said fiducial point inthe third cardiac complex and noting a corresponding third cardiaccomplex fiducial point time; and measuring voltages of the fourthcardiac complex at times using said measurement intervals taken relativeto the third cardiac complex fiducial point time.
 17. The method ofclaim 16, comprising: measuring voltages of the second cardiac complexat the measurement intervals; forming a first vector from the measuredvoltages of the second cardiac complex; forming a second vector from themeasured voltages of the fourth cardiac complex; and comparing the firstand second vectors.
 18. The method of claim 17, further comprisingdetecting whether a tachyarrhythmia is present, and wherein detectingthe first and second cardiac complexes is carried out in the absence ofthe tachyarrhythmia, and wherein detecting the third and fourth cardiaccomplexes is carried out in the presence of the tachyarrhythmia, andfurther comprising classifying the tachyarrhythmia as a supraventriculartachyarrhythmia (SVT) or a ventricular tachyarrhythmia (VT) using thecomparing of the first and second vectors.
 19. The method of claim 18,wherein comparing the first and second vectors comprises computing acorrelation coefficient between the first and second vectors, andwherein classifying the tachyarrhythmia includes classifying thetachyarrhythmia as SVT if the correlation coefficient exceeds apredetermined threshold and classifying the tachyarrhythmia as VT if thecorrelation coefficient is less than or equal to the predeterminedthreshold.
 20. The method of claim 16, wherein establishing measurementintervals comprises: defining a reference time at a specified intervalfrom the first cardiac complex fiducial point time; and defining themeasurement times relative to the reference time.